Rosetta was today inserted in a large orbit around 67P . Congrats to Esa and all other involved agencies ! In the months to come Esa will look for an appropriate landing site for the lander . Then the hard job may come to set the lander to the ground . Given te complex geomtery of the comet this may require some good navigation skills . Suppose Esa decides to land in the "neck" of the comet . How to do this ?

I've set up a simple GravSim of the geometry of the comet consisting of a very close binary of two small bodies . Masses and dimensions are in the order of magnitude of 67P . In this sim the revolution period is about 3 hours , where in reality the period is about 12 hours. Rosetta is represented as the small yellow probe orbiting at about 20 km as the sim starts . Enjoy the landing ...using the navigition bottons !

After some trial and error ( at which the probe crashes ) it is even possible to navigate through the narrow gap between the simulated 2 major bodies. The trick is to trottle "towards" and "away " alternatively . In this way the probe stays more or less "stationary" while the comet is rotating .

Here's a new updated sim which is more accurate : dimension 67P : 4 km , divided into 2 bodies , first being 2400 m , the second 1600 m . Density was choosen to 2000 kg/m³ which may be exagerated . Maybe 1000 kg/m³ may be a better figure . The rotation speed then becomes ca. 6 hours which still is to big . I added Rosetta at sma 7500 meter from the bigger part .

The sim hereunder is the most accurate I could get till now . The density of the comet was set to 350 kg/m³ which gives a revolution period of about 12 hours , which corresponds highly to the measured value by Rosetta (12.6 h ) .

Yet the comet is still simulated as a pair of two bodies , a very close binary . This means in this sim nothing holds the comet together , except mutual attraction . This also means the comets consistency must be very very loose in reality , as the orbital periods more or less match . If the comet would rotate faster there is a good chance it might break in two ...

In the sim I added several possible orbits of the spacecraft. The most stable orbit seems to be a polar orbit ( over the main body ) .

Unfortunately , due to the low mass of the comet ( in this sim being 3.512 E+12 kg ) and therfor low gravitational filed I was not able anymore to land on the comet using the trusters , because the trusters are too powerfull . It is not easy to manoeuver in such a weak field. The escape velocity is about 0.45 m/s !!

@ Tony : is there a method to lower the thrust in Gravsim , so that one can attempt a landing in this configuration ?

The sim shows 3 coplanar orbits of Rosetta at about 5000 m initial distance , and one ( the blue orbit ) a polar orbit , also at 5000 m from the bigger part of the comet . The picture shows the result after 10 days orbiting . Clearly visible is the precession the comet will give on the orbit of Rosetta.

When I first saw the animation of the comet spinning a few weeks ago, I did a quick calculation and found as you did: The comet is spinning fast enough that it is on the verge of breaking up. So modeling it at two orbiting lobes is a good idea. Try including the Sun in your sim. This comet is of low enough mass that the Sun might also heavily perturb the orbit of the spacecraft.

The comets extraordinal shape did me wonder about the potential field this body has.

Out of interest I've created a simple model of the comet , consisting of two spheres , and did some excel modelling . The picture of this model in annex gives the gravitational potential along the long axis of the comet . I generated an envelope of 100 m above the surface of the comet and calculated the potential energy . As can be seen the "head of the duck" has the maximum potential . The neck has the lowest potential energy . This means the neck should be the most stable region . This makes sense by analogy ( valleys )

When I first saw the animation of the comet spinning a few weeks ago, I did a quick calculation and found as you did: The comet is spinning fast enough that it is on the verge of breaking up. So modeling it at two orbiting lobes is a good idea. Try including the Sun in your sim. This comet is of low enough mass that the Sun might also heavily perturb the orbit of the spacecraft.

Yes , I think it must be at the edge of breaking down , depending upon the rotation axis . If it rotates along the long axis there may not be a problem .. I'm sure a debate about this will follow soon .

Concerning adding the sun in the sim : I can do this , but I think Rosetta is now well inside Hills Sphere which I calculated to be about 430 km . Rosetta now is at 100 km .

Tony : It would also interest me how I could lower the "thrust" in the thrust boxes in GravSim . Is there a way to get a thrust of let's say 1/10 or 1/100 of the actual minimum value provided ?

This means that if we simulate the comet as a binary the orbital period would become about 18.8 hours (taking r head = 800 m ; r body = 1200 m) . The binary model doesn't work anymore as the orbital period is only 12.5 hours . This also means that the comet is held together by other forces , other than gravitation alone , as we could expect

This is funny , reopening the same article on the blog now gives a mass of 10^13 kg , this is 10 times more than a day ago . Must have been a typo which they corrected . So the density may be order of 1100 kg/m³ . This is good news . The comet is better protected from flying apart .

Here's a simulation program , written in PoweBasic I made recently in order to visualize the difficulties the ESA's team might have to land their Philae Lander safely onto Comet 67P . I first tried to do this in GravSim , but was'nt succesfull because of the comets complex gravity field. In the sim the comet is simulated being two different point masses , rotating around a common barycenter with period of 12.6 hours . I couldn't match the period of these point masses and the magnitude of the masses and their distance in GravSim . The sim lets you navigate the Rosetta Spacecraft around 67P by the "NAV" button ( click ) . Further the probe can be released , and reattached again. . Feel free to play with it . The Help menu might be usefull . I tried to make ths sim as realistic as possible ( best guesses for masses , distances...aso ) , so it visualizes in 2D well what Rosetta has to expect at 12/11/2014 . You can unzip the folder , keep the files together and press the .exe . All the rest is just clicking the right buttons. The .bas file contains the code which is compilable in a PBWin10 compiler .

Although it's not a GravSim code I thaught I might share it here for those interested.